This invention relates generally to a device for compensation of chromatic dispersion in optical fiber communication systems.
Most high-speed fiber optic communication systems today use externally modulated lasers to minimize laser xe2x80x98chirpxe2x80x99 and reduce the effects of chromatic dispersion in the fiber. Even with external modulation, there is a certain amount of xe2x80x98chirpxe2x80x99 or broadening of the laser spectrum, because any modulated signal must contain frequency xe2x80x98sidebandsxe2x80x99 which are roughly as wide as the modulation rate. Higher bit rate transmission systems consequently have broader frequency sidebands, and at the same time can tolerate less phase delay because of the shorter bit period. Next-generation high bit rate systems are consequently very sensitive to chromatic dispersion of the optical fiber and any components such as WDM""s within the system.
Chromatic dispersion of optical fiber is roughly constant over the 1550 nm communication window, and can be compensated by several techniques including dispersion compensating fiber, FBG""s, etc. However, certain wavelength filtering components such as WDM""s can have significant dispersion characteristics due to a fundamental Kramers-Kronig type relationship between transmission spectrum and dispersion characteristics. This type of dispersion characteristic typically varies substantially over the narrow WDM passband, and therefore is difficult to compensate using conventional techniques such as dispersion compensating fiber. It is one objective of the present invention to compensate for the dispersion from WDM devices, including multiplexers, demultiplexers, and interleavers. Conventional laser systems are known to utilize directly modulated semiconductor lasers. In combination with chromatic dispersion characteristics of single-mode optical fiber, chirping of these lasers contributes to the spread of optical pulses and results in intersymbol interference and overall degradation in transmission. Current and xe2x80x9cnext-generationxe2x80x9d high speed systems employ transmitters which use narrow linewidth lasers and external modulators in a window or range of wavelengths about 1550 nm. These external modulators generally have a very low chirp; some designs have a zero or negatively compensating chirp. Nevertheless, transmission distance is still dispersion-limited to about 80 kilometers at transmission rates of 10 Gb/s using conventional single mode fibers.
One solution to this problem is in the use of dispersion shifted fiber which has little dispersion in the 1550 nm window. However, replacing existing fiber with dispersion shifted fiber is costly. Other approaches have been proposed such as optical pulse shaping to reduce laser chirp, using a semiconductor laser amplifier to impose a chirp on the transmitted signal that counteracts the chirping of the directly modulated semiconductor laser.
Approaches that are more consistent with the teachings of this invention attempt to reduce the intersymbol interference at or near the receiver, or intermediate the transmitter and the receiver. Essentially any medium capable of providing a sufficient dispersion opposite to that of the optical fiber can serve as an optical pulse equalizer. For example it is known to use a special optical fiber having an equal chromatic dispersion at a required operating wavelength but opposite in sign to that of the transmitting fiber. Other methods include the use of fiber Bragg gratings as disclosed in U.S. Pat. No. 5,909,295 in the name of Li et al., and disclosed by Shigematsu et al., in U.S. Pat. No. 5,701,188 assigned to Sumitomo Electric Industries, Ltd., and the use of planar lightwave circuit (PLC) delay equalizers. Unfortunately, special compensating fiber has a very high insertion loss and in many applications is not a preferable choice. Fiber gratings are generally not preferred for some field applications due to their narrow bandwidth, and fixed wavelength. PLCs are also narrow band, although tunable devices; fabricating a PLC with large dispersion equalization remains to be difficult. Shigematsu et al. disclose a hybrid of both of these less preferred choices; dispersion compensating fibre with chirped Bragg gratings.
In a paper entitled xe2x80x9cOptical Equalization to Combat the Effects of Laser Chirp and Fiber Dispersionxe2x80x9d published in the Journal of Lightwave Technology. Vol. 8, No. 5, May 1990, Cimini L. J. et al. describe an optical equalizer capable of combating the effects of laser chirp and fiber chromatic dispersion on high-speed long-haul fiber-optic communications links at 1.55 xcexcm. Also discussed is a control scheme for adaptively positioning the equalizer response frequency. Cimini et al. describe a device having only one common input/output port at a first partially reflective mirror and a second 100% reflective mirror together forming a cavity. The control scheme described attempts to track signal wavelength by obtaining feedback from a receiver. The amplitude response of the equalizer is essentially flat with wavelength at the input/output port, and thus, the proposed control scheme is somewhat complex requiring processing of high speed data at the optical receiver. As well, the proposed control method is stated to function with RZ signals but not with NRZ signals, a more commonly used data format. Although the equalizer described by Cimini et al. appears to perform its intended basic dispersion compensating function, there exists a need for an improved method of control of the position of the equalizer frequency response, and as well, there exists a need for an equalizer that will provide a sufficient time shift over a broader range of wavelengths. U.S. Pat. No. 5,023,947 in the name of Cimini et al., further describes this device.
A Fabry-Perot etalon having one substantially fully reflective end face and a partially reflective front face is known as a Gires-Tournois (GT) etalon. In a paper entitled Multifunction optical filter with a Michelson-Gires-Turnois interferometer for wavelength-division-multiplexed network system applications, by Benjamin B. Dingle and Masayuki Izutsu published 1998, by the Optical Society of America, a device is described which is hereafter termed the MGT device. The MGT device as exemplified in FIG. 1 serves as a narrow band wavelength demultiplexor; this device relies on interfering a reflected E-field with an E-field reflected by a plane mirror 16. The etalon 10 used has a 99.9% reflective back reflector 12r and a front reflector 12f having a reflectivity of about 10%; hence an output signal from only the front reflector 12f is utilized.
In an article entitled xe2x80x9cOptical All-Pass Filters for Phase Response Design with Applications for Dispersion Compensationxe2x80x9d by C. K. Madsen and G. Lenz, published in IEEE Photonics Letters, Vol 10 No. 7, July 1998, a coupled reflective cavity architecture in optical fibre, as shown in FIG. 19, is described for providing dispersion compensation. Cavities are formed in the optical fiber between fiber Bragg grating reflectors. However a multi-cavity filter in fiber has a limited free spectral range (FSR) insufficient for a telecommunications system. For a typical 100 GHz FSR required in the telecommunications industry, the cavity length is about 1 mm. A Bragg grating reflector, if manufactured using commonly available grating -writing techniques, would need to be longer than 1 mm, and hence the two reflector cavity structure would be too long to achieve the necessary FSR. Another draw back to this prior art solution is the requirement for an expensive optical circulator to separate the input and output signals.
As of late, interleaving/de-interleaving circuits are being used more widely. These specialized multiplexor/demultiplexors serve the function of interleaving channels such that two data streams, for example a first stream consisting of channel 1, 3, 5, 7, and so on, is interleaved, or multiplexed with a second stream of channels, 2, 4, 6, 8, and so on, for form single signal consisting of channels 1, 2, 3, 4, 5, 6, 7, 8, and so on. Of course the circuit can be used oppositely, to de-interleave an already interleaved signal, into plural de-interleaved streams of channels. One such interleaver circuit is described in U.S. Pat. No. 6,125,220 issued Sep. 26, 2000 in the name of Copner et al., and another is in U.S. Pat. No. 6,040,932 issued Mar. 21, 2000 in the name of Colbourne et. al. Although interleaver circuits perform a desired function, it has been discovered that some of these circuits suffer if from unwanted periodic chromatic dispersion within each channel. It is this type of periodic dispersion that can be obviated or lessened by the instant invention. It should also be noted that in many instances it is not desirable to completely eliminate all chromatic dispersion; it is believed that a small amount of such dispersion can be useful in reducing non-linear effects in WDM systems; therefore, the instant invention can be used to lessen dispersion by a required amount.
Hence, it is an object of this invention to overcome some of the limitations of the prior art described above. Furthermore, it is an object of the invention to provide a passive device that will compensate for or lessen dispersion over a plurality of interspaced wavelength channels simultaneously.
In accordance with the invention there is provided, a method of dispersion compensation for simultaneously compensating for dispersion present within individual channels in a multi-channel system having a multi-channel signal, having a predetermined channel spacing between adjacent channels, the method comprising the steps of: providing a GT resonator including at least two optical cavities; and, launching the multi-channel signal into the GT resonator at a first angle and capturing a return signal from the GT resonator at a second angle.
In accordance with the invention there is further provided, a method of simultaneously providing dispersion compensation within a plurality of channels in a multi-channel optical signal having predetermined channel spacing between adjacent channels, comprising the steps of: providing the multi-channel optical signal requiring dispersion compensation of individual channels within the optical signal;
launching the optical signal at a first angle into a periodic device having a substantially unchanging amplitude output response with respect of the wavelength of the optical signal, the periodic device having an output response which varies periodically in phase with respect to wavelength of the optical signal, the periodically varying phase having a correspondence with the channel spacing; and,
receiving a dispersion compensated output signal at a second angle having its dispersion adjusted by the periodic device in a periodic manner which has a correspondence to the channel spacing.
In accordance with the invention there is further provided, a method of dispersion compensation of individual channels simultaneously in a multi-channel signal having a predetermined channel spacing between adjacent channels, wherein the GT resonator has a free-spectral range that is equal to or that is an integer multiple of or integer fraction of the channel spacing of the multi-channel optical system.
In another aspect of the invention there is provided, a dispersion compensation device for compensating a multi-channel optical signal having a channel spacing which is periodic, comprising: a multi-cavity etalon having at least one end face that is highly reflective and substantially not transmissive to light and at least two other faces that are partly reflective and partly transmissive, the one and face and the at least two other faces being separated from one another by predetermined gaps, the at least three faces forming at least two resonating cavities.
In accordance with an alternative embodiment of the invention in a multi-channel optical system, having a multiplexed multi-channel optical signal wherein the optical signal has periodic dispersion repeating in each channel, the channels being spaced in periodic manner, wherein the distance between centre wavelengths of adjacent channels is predetermined and a channel width is predetermined, there is provided, a method of simultaneously compensating for the periodic dispersion in each channel comprising the steps of:
launching the optical signal into an optical device having an input/output response in amplitude that is substantially unchanging, and that has an input/output response that varies in dispersion periodically and substantially oppositely to the periodic dispersion repeating in each channel of the multi-channel optical signal.
In accordance with the invention there is provided, an optical system for providing interleaving of optical channels into an optical signal and for providing dispersion compensation of the interleaved optical channels, comprising:
an interleaver optical circuit for interleaving the channels into the optical signal; and,
a multi-cavity etalon, one of the cavities being a GT cavity, at least one of the cavities having a free spectral range that corresponds with channels spacing of the interleaved optical channels, the multi-cavity etalon having a periodic dispersion characteristic that is substantially opposite in slope to the slope of periodic dispersion within the channels within the optical signal, the multi-cavity etalon being disposed to receive the optical signal from the interleaver optical circuit, the multi-cavity etalon having an amplitude response that is substantially unchanging over wavelengths corresponding to the interleaved channels.
Advantageously, the multi-cavity etalon in accordance with the present invention has an FSR in the range of 25 GHz to 400 GHz, suitable for telecommunications applications.